If you want to re-pot then wait until it has finished 'flowering'. But you should remember that trying to get a poinsettia to re-'flower' is *not* an easy task. I remember trying many years ago, and have spoken to many people since who have tried with poor results. I can't honestly remember the exact conditions that were required but I know that temperature and light were both critical. Sorry if this sounds negative but at home we took to the usual answer of just throwing them once 'flowering' finished (I think technically they are brachts not flowers) and getting a new one the following year for Christmas. Anybody that knows me will want to know why I am advocating spending money on plants but believe me, if you don't have exactly the right conditions, which I think only professional growers can afford, growing hundreds or even thousands of them, then you probably have 2 chances - no chance and a cat in Hell's chance. If you want to try, it is so complex, I suggest you Google for all the information you can. If you just want to keep the greenery alive you could try potting on once the brachts die, but I don't think it is worth it, it is only the brachts that are impressive.

Sad to say I agree with Sarraceniac keeping your plant going and getting it to produce the red bracts again the following year is a challenge. Poinsettias are arid, subtropical plants needing good air circulation and excellent drainage to thrive.

Do not overwater, the compost surface should be slightly dry before you water again. However, when you do water, water the plant thoroughly and ensure free drainage. Do not let the compost dry completely.

I find you need to take care when buying, they often come in clear wrappers, and its worth checking than on removing it, it is not full of leaves as they start dropping them so quickly, sometimes the cheap supermarket ones are best as they have a quick turnover. Like veg!!

They hate getting cold too, a friend bought a nice one the other year from the supermarket, it was cold and windy I found out later and after a day or so all the leaves dropped off

On the other hand, if the room temperature goes much over 21º C, it will drop all of its green leaves, and the red leaves will gradually follow.